A conventional organic EL element is shown by a reference numeral 101 in FIG. 6.
This organic EL element 101 possesses a substrate 102. An anode electrode layer 103, a hole injection layer 104, an organic light-emitting portion 105 and an electron injection layer 106 and a cathode electrode layer 107 are formed on the substrate 102 in this order.
When a positive voltage and a negative voltage are applied to the anode electrode layer 103 and the cathode electrode layer 107, holes injected into the organic light-emitting portion 105 from the hole injection layer 104 and electrons injected into the organic light-emitting portion 105 from the electron injection layer 106 are recombined inside of the organic light-emitting portion 105; and the organic light-emitting portion 105 emits a light.
In case of the top-emission-type organic EL element, the anode electrode layer 103 has a high reflectance, and the cathode electrode layer 107 is composed of a material having a high transmission. Thus, the light radiated toward the cathode electrode layer 107 from the organic light-emitting portion 105 is emitted outside the organic EL element 101 through the cathode electrode layer 107, while the light radiated in a direction of the anode electrode 103 is reflected by the anode electrode layer 103, and is emitted outside through the organic light-emitting portion 105 and the cathode electrode layer 107.
Therefore, in the case of the top-emission-type organic EL element, it is required that the cathode electrode layer 107 not only has a high transmission but also a low resistivity, which is needed to pass a current uniformly in a plane so as to uniformly emit the light in the plane.
As an electrode material which satisfies such requirements, there is a thin film of an MgAg alloy. A cathode electrode layer 107 composed of the MgAg alloy is formed by emitting an Mg vapor and an Ag vapor separate from an Mg vapor generating source and an Ag vapor generating source according to a vapor deposition method and bringing them together to a surface of an electron injection layer 106.
However, although omitted in FIG. 6, since there are actually steps on a surface of the organic EL element 101 and the vapor deposition method exhibits a poor step coverage, there is a problem in that disconnection is formed in the cathode electrode layer 107 at shadowed portions around the projected portions of the steps.
On the other hand, when an MgAg thin film is to be formed by sputtering an MgAg target, the electron injection layer 106 is subjected to an argon plasma.
The electron injection layer 106 is a very thin film made of an electron injection material (such as, Li (lithium), LiF (lithium fluoride), cesium fluoride CsF), so that there is a problem in that when the electron injection layer is exposed to the plasma, it is sputtered and disappears from on the organic light-emitting portion 105.
Organic EL elements using MgAg thin films are described in, for example, the following documents:
Patent Document 1: JP-A 2006-344497,
Patent Document 2: JP-A 2004-327414,
Patent Document 3: JP-A 2001-217074.